Arsenic doped polysilicon was deposited on a <001> oriented Si wafer. Cross-section TEM specimen preparation was performed, giving the vertical structure of the stack. The observation was done with a <110> zone axis to ensure that the interface is strictly oriented under the electron beam. High resolution imaging on the very interfacial region proved the existence of an ultrathin regular amorphous layer running along the interface (thickness 1 rim). Complementary analytical investigations showed that the layer was really amorphous (EELS) and that the dopant was overconcentrated in this layer (EDXS linescan and SIMS profile). Therefore, a slight amorphization of the interface added to an abnormal migration of the dopant gives a well detectable layer separating both crystalline phases.